On auxiliary power units (APUs) with aft bearings, there is a requirement to pass lubrication into the bearing, scavenge the lubrication out, and vent the bearing compartment. This is conventionally done using tubes that are attached to the bearing housing and pass through the exhaust duct to the exterior of the APU engine where the tubes can be connected to further tubes, pumps and the like.
A particular design of tubes for lubricating APU bearings utilizes a threaded end fitting that is screwed into the bearing compartment and is fitted with a sheet metal seal. Problems arise in this design because the tubes need to be locked into place, torque wise, so they do not become unthreaded and thus no longer attached to the bearing. At the same time, there must be a way to permit relative expansion between the relatively cool tubes and the very hot exhaust duct, of perhaps 1200° F. (649° C.) to 1600° F. (871° C.). In addition, vibration of the tubes needs to be dampened to prevent tube damage.